Generally, adhesion of pigment particles of red, green and blue color to the respective surfaces of the corresponding red, green and blue phosphors employed in manufacturing a luminescent screen of a color cathode ray tube results in a clearer emission color, since a light having undesired wavelength within the emission spectra is absorbed or attenuated due to the filter effect of these pigment particles adhering thereto.
Luminescent screen formed by adding these pigments enhances the absorption of incident external light by the pigment coloration so as to reduce the reflectance of light from a luminescent screen, and therefore the contrast of the image is enhanced highly. .alpha.-Fe.sub.2 O.sub.3 is used widely as red pigment. Methods for coating a pigment in pigment coated phosphor which coats a pigment particle on the surface of a phosphor particle are as follows. (see U.S. Pat. No. 3,275,466 and U.S. Pat. No. 4,152,483)
1st method comprises the steps of (1) dispersing pigment particles into an aqueous solution of a resin such as polyvinyl pyrrolidone (PVP), (2) dispersing phosphor particles into a gelatin solution, (3) mixing the above two dispersions and stirring them thoroughly and (4) drying the resulting precipitates.
2nd method uses an acidic polymer and a basic polymer, which are water-soluble, instead of said PVP and gelatin.
3rd method comprises the steps of (1) preparing an aqueous solution of gelatin and an aqueous solution of gum arabic and homogeneously dispersing a phosphor into one of these aqueous solutions, (2) homogeneously dispersing pigment particles into the other solution and (3) mixing the two dispersions.
When a pigment coated phosphor is manufactured by said method, it tends to cause the flocculation during adhesion of pigment and the dispersion and affinity for a phosphor slurry is bad, and thus, this is disadvantageous in that all the characteristics is deteriorated if a luminescent screen of stripe or dot pattern is formed on the inner surface of the panel.
In this respect, the pigment coated phosphor has to meet the following conditions in order to have an excellent contrast:
1) the adhesion between the pigment particle and the phosphor particle should be strong, so that there should be no detachment of the pigment particle from the surface of the phosphor during dispersing in the slurry;
2) the pigment particles should be uniformly spread on the surfaces of the phosphor particles, and the flocculation of the phosphor particles should be negligible;
3) the pigment particles should be volatile and dispersed to have no foreign material when the material to coat said pigment particles is baked during the formation of a luminescent screen.